As is known, in the transmission between the gear box and the wheels of a four wheel drive motor vehicle there is normally fitted an epicyclic mechanism which acts to transmit the drive from the engine shaft to a front shaft which drives the front wheels and to a rear shaft which drives the rear wheels; the transmission further includes a rear differential and a front differential the first of which connects the said rear shaft with the rear half-shafts while that at the front connects the front half with the front half-shafts.
The epicyclic mechanism serves to divide the torque between the front shaft and the rear shaft in a predetermined torque ratio when the vehicle is in normal operating conditions travelling forwards in a straight line and allows each of the front and rear shafts to assume the angular velocity imposed on them by the rolling conditions of the wheels of the motor vehicle.
In a transmission of this type, whenever the wheels of the front or rear axles reach the slip threshold condition because of the reduction in adhesion between the wheel and the ground, the torque tends to be transferred only to the slipping wheel which is not able to transmit any tangential force to the ground, whilst at the other wheel of the same axis (which is not slipping) the torque tends to fall to zero; consequently no tangential force able to cause displacement of the motor vehicle is transmitted from the wheels of that axle. The same disadvantage occurs whenever both the front wheels or both the rear wheels reach the slip threshold condition.
For the purpose of eliminating this disadvantage in transmissions of this type there are normally provided suitable locking devices which have the purpose of locking one of the two differentials or the epicyclic mechanism when one of the above-indicated slip threshold conditions is reached; these locking devices normally comprise clutches which are able rigidly to connect two rotary members of the differential or the epicyclic mechanism and are controlled by suitable servo mechanisms actuated, for example, electromagnetically. Activation of these latter is controlled by a suitable electronic central processing unit when the predetermined operating conditions are reached.
Transmissions of this latter type, although they obviate the disadvantages referred to initially, are constructionally very complex and therefore expensive, require maintenance and in many cases are not very reliable.